GCS_MAVLink: rename ch variable

avoid conflict with ch variable from ChibiOS

libraries/GCS_MAVLink/GCS_MAVLink.h

@@ -50,12 +50,12 @@ static inline bool valid_channel(mavlink_channel_t chan)
 /// @param chan		Channel to send to
 /// @param ch		Byte to send
 ///
-static inline void comm_send_ch(mavlink_channel_t chan, uint8_t ch)
+static inline void comm_send_ch(mavlink_channel_t chan, uint8_t chr)
 {
     if (!valid_channel(chan)) {
         return;
     }
-    mavlink_comm_port[chan]->write(ch);
+    mavlink_comm_port[chan]->write(chr);
 }
 
 void comm_send_buffer(mavlink_channel_t chan, const uint8_t *buf, uint8_t len);